The present invention relates to time-temperature indicators, particularly to time-temperature indicators of the type utilizing thermochromic material. More particularly, it relates to time-temperature indicators for use with closed or sealed heating or cooking vessels such as pressure cookers and sterilizers or lids on pots and pans.
The use of different types of mechanical and electrical timers to indicate how long something has heated or how long a pressure cooker, for example, has been cooking, are well known. However, when more than one vessel or cooker is in operation simultaneously, it becomes difficult to time all these vessels with only one timer. Besides, these prior art timers are expensive and cannot compensate for varying boiling points at different altitudes.
Temperature indicating devices using thermochromic material are known in the prior art. These devices utilize thermochromic material in which the material goes through a spectrum of color changes at different temperatures. These prior approaches incorporate thermochromic materials deposited in the shape of numbers or are sheet material which gives out readouts or color changes at predetermined temperatures depending on the precise composition of the material. However, these prior known indicators are not capable of determining internal temperatures of a vessel or pot after a given length of time.
Another prior art effort using thermochromic materials as a time-temperature indicator is described and claimed in U.S. Pat. No. 4,137,769 issued Feb. 6, 1979 to R. Parker. It involves a method and apparatus for determining the internal temperature of an object such an egg. The thermochromic material of that indicator is enclosed within a transparent member having physical dimensions, thermal conductivity and diffusivity characteristics selected to provide a thermal analog of the associated object being tested. The indicator has also to be immersed in the heating fluid. In addition, indicators of this type must be placed within or inside an open vessel or in one with a transparent top, along with the object being checked or tested. Such indicators, therefore, cannot be utilized for applications such as heating or heated vessels or pressure cookers and the like where the material therein is not visible from the outside or exterior of the vessel.
Thus a need exists for a time-temperature indicator by which one can readily determine the temperature of the material within a sealed or closed vessel after a finite period of time.